1. Field of the Invention
The invention relates generally to the field of power systems, and, more particularly, to a Quasi-Isothermal Brayton Cycle power system.
2. Description of the Related Art
For mobile applications, such as an automobile or truck, generally it is desirable to use a heat engine that has the following characteristics:                Internal combustion to reduce the need for heat exchangers;        Complete expansion for improved efficiency;        Isothermal compression and expansion;        High power density;        High-temperature expansion for high efficiency;        Ability to efficiently “throttle” the engine for part-load conditions;        High turn-down ratio (i.e., the ability to operate at widely ranging speeds and torques;        Low pollution;        Uses standard components with which the automotive industry is familiar;        Multifuel capability; and        Regenerative braking.        
There are currently several types of heat engines, each with its own characteristics and cycles. These heat engines include the Otto Cycle engine, the Diesel Cycle engine, the Rankine Cycle engine, the Stirling Cycle engine, the Erickson Cycle engine, the Carnot Cycle engine, and the Brayton Cycle engine. A brief description of each engine is provided below.
The Otto Cycle engine is an inexpensive, internal combustion, low-compression engine with a fairly low efficiency. This engine is widely used to power automobiles.
The Diesel Cycle engine is a moderately expensive, internal combustion, high-compression engine with a high efficiency that is widely used to power trucks and trains.
The Rankine Cycle engine is an external combustion engine that is generally used in electric power plants. Water is the most common working fluid.
The Erickson Cycle engine uses isothermal compression and expansion with constant-pressure heat transfer. It may be implemented as either an external or internal combustion cycle. In practice, a perfect Erickson cycle is difficult to achieve because isothermal expansion and compression are not readily attained in large, industrial equipment.
The Carnot Cycle engine uses isothermal compression and adiabatic compression and expansion. The Carnot Cycle may be implemented as either an external or internal combustion cycle. It features low power density, mechanical complexity, and difficult-to-achieve constant-temperature compressor and expander.
The Stirling Cycle engine uses isothermal compression and expansion with constant-volume heat transfer. It is almost always implemented as an external combustion cycle. It has a higher power density than the Carnot cycle, but it is difficult to perform the heat exchange, and it is difficult to achieve constant-temperature compression and expansion.
The Stirling, Erickson, and Carnot cycles are as efficient as nature allows because heat is delivered at a uniformly high temperature, Thot, during the isothermal expansion, and rejected at a uniformly low temperature, Tcold, during the isothermal compression. The maximum efficiency, ηmax, of these three cycles is:       η    max    =      1    -                  T        cold                    T        hot            
This efficiency is attainable only if the engine is “reversible,” meaning that the engine is frictionless, and that there are no temperature or pressure gradients. In practice, real engines have “irreversibilities,” or losses, associated with friction and temperature/pressure gradients.
The Brayton Cycle engine is an internal combustion engine that is generally implemented with turbines, and is generally used to power planes and some electric power plants. The Brayton cycle features very high power density, normally does not use a heat exchanger, and has a lower efficiency than the other cycles. When a regenerator is added to the Brayton cycle, however, the cycle efficiency is increased. Traditionally, the Brayton cycle is implemented using axial-flow, multi-stage compressors and expanders. These devices are generally suitable for aviation in which aircraft operate at fairly constant speeds; they are generally not suitable for most transportation applications, such as automobiles, buses, trucks, and trains, that must operate over widely varying speeds.
The Otto cycle, the Diesel cycle, the Brayton cycle, and the Rankine cycle all have efficiencies less than the maximum because they do not use isothermal compression and expansion steps. Further, the Otto and Diesel cycle engines lose efficiency because they do not completely expand high-pressure gasses, and simply throttle the waste gasses to the atmosphere.